phy53 lecture dr. brown 4 november 2010
more gravity
how do we know G = 6.67 x 10^{-11} N m^2/kg^2 ?
measuring G is equivalent to measuring M_earth
Cavendish Experiment
restoring torque from torsional pendulum \tau = -K \theta
[recall F_spring = -k x]
F_g -- torque balance
\frac{2GMmL}{d^2} = K \theta ... solve for G
Tides
consider two equal masses lined up radially
neap and spring tides
lunar dominates solar contribution
Humans Made of water -- so let us study ... Fluids!
microscopic view to start...
pressure = F/(unit area)
[perpendicular to the fluid]
far too many calculations to apply N2 to every molecule...get average/macroscopic picture...
define density \rho = \frac{dM}{dV} V = volume
Gas -- vs -- Liquid
[both fluids]
Gas :
relatively low density
weakly interacting
spread out to fill container
compressible
Liquid :
relatively high density
strongly interacting
confined by container, retain cohesion ( surface tension)
relatively incompressible
density of water
\rho_w = 1000 kg/m^3
specific gravity = density of material / density of water
consider glass of water
Pressure of air = 1 atm = 10^5 Pa SI units Newtons/meter^2 = Pascals
P(z) = \rho g z + P_air
example -- breathing through reed 2m below surface of water (very difficult)